1. Field of the Invention
The invention relates to read-only memory devices, and more particularly to a new programmable read-only memory (PROM) using a diode structure and a method of fabricating same.
2. Description of the Prior Art
Read-only memories, which comprises an array of memory cells, are widely utilized in digital electronic equipment. For example, computer systems including microcomputers and minicomputers use read-only memories for storing fixed software/firmware routines. A programmable read-only memory (PROM) is a fixed, non-volatile memory. Normally, a PROM's memory cells are pre-programmed with specific data at the time the PROM is manufactured and before the PROM is delivered to a customer. The fabrication process for PROM devices is complicated and requires sophisticated processing steps, each of which consumes precious manufacturing time for material processing and for adjusting manufacturing parameters.
The manufacturing process steps for most compatible PROM devices are virtually the same, up to the point where each PROM is programmed with its respective memory contents. Thus, it is possible to manufacture PROM devices to a semi-finished stage and store them until they are needed to be programmed with designated memory contents and then are promptly delivered to the customer at their request. Such "post-programmed" mask PROMs are commonly employed in the art of PROM manufacturing.
In the programming stage, a commonly used technique to program the memory cells in the PROM selectively implants impurities into predetermined memory cells to adjust their threshold voltage, so as to set them permanently in an ON state or in an OFF state. However, it is hard to precisely control the implanting energy and dosage of the impurities in scaled down process techniques. Moreover, a mask is needed to implant only the cells which are programmed, and that fact increases the complexity of manufacture.
Another prior art programming method forms a dielectric layer at a position within metal contacts of selected memory cells. The dielectric layer acts as a control layer. Thus, memory cells with the control layer are permanently set in an OFF state, while memory cells without the control layer are permanently set in an ON state. This programming process will be better understood from the following detailed description. Turning to FIG. 1, an active region is defined by forming a field oxide layer 11 on a silicon substrate 10 using a conventional LOCOS process. AMOS transistor comprising a polysilicon gate electrode 12 (word-line) and source/drain regions 13 (bit-lines) is formed in the active region and serves as a memory cell of the PROM device. A dielectric layer 14 is deposited overlying the MOS transistor, and metal contacts are formed therein to expose the underlying source/region regions. A control layer 15, such as a silicon dioxide layer or a silicon nitride layer, is next formed within the metal contact of selected memory cells. Finally, a metal line 16 is formed on the dielectric layer 14 completing the programming process of the prior art PROM device.
FIGS. 2A and 2B are schematic circuit diagrams showing the OFF state and ON state of the prior art PROM device, respectively. The conducting metal line 16 and source/drain regions 13 and the dielectric control layer 15 together function as a capacitor. Thus, the MOS transistor with control layer 15 is not conductive, i.e. assumes an OFF state, as shown in FIG. 2A. On the other hand, the MOS transistor without the control layer 15 is conductive, i.e. assumes an ON state, as shown in FIG. 2B.
However, in the prior art programming process a mask is needed to define the control layer 15, which increases the complexity of processing and reduces the efficiency of manufacture. Besides, since a MOS transistor is used to serve as a memory cell and a field oxide layer 11 is formed to isolate the MOS transistors, it is hard to further reduce the memory cell size. Hence, the prior art PROM device is not well suited for scaled down processing.